A device for warning a degree of potential risk is known which issues a warning in accordance with the level of possibility that a subject vehicle and another vehicle traveling in an adjacent lane come close to each other when the subject vehicle and the other vehicle travel side by side along a curve (Japanese Patent Application JP2007-148964A).
In automated or autonomous travel control, however, a problem may arise that, when the subject vehicle and another vehicle travel side by side along a curve, the driver may feel uneasy.